Re: Signal isn't an easy option
Not being on WhatsApp just means that my children are likely to miss out on informal activities arranged through it.
So in practice you are being required to use WA in order for your children to take part. That's illegal.
The PTA does not make it a requirement but for some discussions outside of formal meetings WhatsApp does get used.
Ditto. That's illegal. Actually, depending on what is being discussed on WA, the actual use may well be illegal regardless of the "use it or miss out" issue.